The present invention is directed to electro-optical display devices for optically representing items of information such as digits, characters and the like, the display device having front and rear plates which are spaced apart to provide a chamber for receiving a switchable medium such as a liquid crystal layer with the front plate being provided with at least one segmented electrode whose segments are individually energized and the rear plate is provided with a rear electrode assembly so that selected portions of the optical material such as the liquid crystal layer can be optically switched between different optical conditions to form a display.
A liquid crystal display device utilizing segmented electrodes are known in contrast to active display devices do not produce their own light but merely modulate the outside or ambient light. Such display devices can be constructed in various different ways and have been commercially available for years in a number of different designs. A review of the possible display principles is given in an article by H. E. Bergt and K. H. Walter in Elektronik, Vol. 4, 1976, pp. 36-42, and a summarized review is given in "Nachrichtetechnische Zeitung" May 28, 1975, pp. k62-k63.
Even with careful production of the display device, it is not possible to prevent individual segments from failing to be energized when they are supposed to be energized during operation or from becoming erroneously activated during the course of operation. Defects of this kind, which can have various origins, for example breaks in the conductor path, break down of the individual drive lines, or short circuit in the region of the medium, frequently lead to a mis-display, which cannot be detected by the observer and which, in many types of uses, for example calibrated instrument or measuring device, cannot be permitted.
In active display devices, for example LED display devices, the representations can be checked by a relatively simple procedure. The segment currents are measured and compared with the offered or coded item of information and, if necessary, a false signal will be triggered. However, this type of monitoring cannot be used for passive display devices, in particular liquid crystal display devices in which the segment currents are several orders of magnitude lower and which currents fluctuate relatively considerably. Previously, a remedy for this situation was found in redundant representation such as disclosed in U.S. Pat. No. 4,037,927 or by utilizing more extensive segmentations, which were disclosed in U.S. Pat. No. 4,054,368. However, both these measurements undoubtedly involve considerable extra expenses with respect to production of the display device and/or the circuitry of the display device.